In many electronic monitoring, control, or data processing or handling systems for a great number of applications, there are requirements for input circuits which receive signals from a source or another part of the system, and transmit them on to another part of the system, usually after filtering, shaping or amplifying the signals. An example might be where pulse-type signals are received at a monitoring or control circuit after having been transmitted over a cable from the sensor or other data handling or transmitting device. Often the signal pulses thus transmitted arrive at the monitoring or control circuit after suffering some attenuation, introduction of noise, and possibly also dispersion during transmission; or alternatively, due to the nature of the sensors or other source devices, the pulses are low voltage amplitude requiring wave shaping and amplifications before further transmission to the utilization, monitoring or control device involved in a particular system. Input circuits of this type are generally well known in the prior art.
It is also common to provide testing circuits in conjunction with such electronic systems whereby test pulses can be introduced at some point in the circuitry, so that their detection and receipt at some further point in the system verifies that at least certain portions of the system are in working condition. For example, in the case of a monitoring system involving remote sensors a test system may include additional switching devices associated with the remote sensors and their cables which operate in a test mode to inject pulses simulating the output of a working sensor. If the pulses are transmitted through the cables and the input circuit, including whatever filtering, wave shaping and amplifying functions it may contain, and are then transmitted to detection or utilization circuits, a positive indication of at least partial operability of the system or channel is indicated. However, the provision of self-testing circuits brings certain associated problems. For one thing, the additional components required for the self-test mode add to the initial cost of manufacture of the system. Another disadvantage is that the additional components required for the self-test mode can themselves represent a reliability problem. This is true because, as a general proposition, the greater the number of components in the system, the greater the probability that there will be a failure somewhere in the system. In particular, with respect to components of a self-test system, since they are so intimately associated with the input data path, a failure in a component there could easily have the effect of shorting, opening, or otherwise disrupting the data path or channel it was intended to serve.